1. Field of the Invention
The present invention relates to an electronic device having a driving circuit and more particularly to an electronic device having an illumination circuit.
2. Description of the Related Art
Currently electroluminescent lamps (EL) are employed in a wide variety of household appliances. ELs have the advantages of slim profile, low heat generation, low power consumption, and high fidelity color. For example, some video projectors employ ELs to emit light showing their company marks, to generate brand awareness.
In general, ELs require alternating current (AC) supplied by a power transformation device. As shown in FIG. 1, the conventional video projector has an EL driving chip 300 and an EL 200. Using the EL driving chip 300 has some disadvantages. For example, the EL driving chip 300 is costly. Moreover, when used, the EL driving chip 300 requires an additional protection circuit or must be electrically isolated, to meet safety specifications.
Additionally, the maximum output voltage of the EL driving chip 300 is limited to 100 volts. The brightness of the EL is directly proportional to the output voltage of the EL. An EL can provide more brightness with a relatively higher voltage. As shown in FIG. 2, the brightness of the EL is enhanced when the output voltage supplied by the EL driving circuit is higher. Therefore, the brightness of the EL can be enhanced by increasing the output voltage of the EL driving circuit.
A video projector using an AC lamp comprises an AC lamp device having an AC lamp device and an AC driving unit (or a ballast). The ballast supplies a power higher than a hundred watts to drive the AC lamp. A small portion of the power can be utilized by an illumination display unit (for example, the EL), thereby reducing the cost of the video projector.
FIG. 3 is a graph showing a current of the AC lamp. As shown in FIG. 3, compared with the current required by the EL, the current of the AC lamp can be referred to as a stable AC current. If a portion of the AC current can be utilized to drive the EL indirectly, an additional current source is unnecessary, thereby reducing the cost thereof.
The present invention provides an effective solution for reducing down the cost of the EL driving chip 300 and to enhance the brightness of the EL.